1. Field of the Invention
The present invention relates to a light source apparatus that illuminates light emitted from a light emitting tube upon a predetermined area of a spatial light modulator or the like and can preferably be applied to an image display apparatus or the like. In addition, the present invention relates to an image display apparatus comprised of the same.
2. Description of the Related Art
In an image display apparatus such as a so-called liquid crystal projector device, a small-sized ultra high pressure mercury lamp having a high brightness is employed as a light source to illuminate a liquid crystal panel as a spatial light modulator.
Since the ultra high pressure mercury lamp offers substantially a point source of light, light emitted from the lamp can be efficiently utilized. In addition, the emission spectrum of the light emitted from the ultra high pressure mercury lamp can be continuous by keeping a pressure in the tube of the lamp from 150 atmospheres (atm) to 200 atm. Furthermore, since the ultra high pressure mercury lamp has a high efficiency of converting the energy applied thereto to visible light (a wavelength range from 400 nanometers (nm) to 700 nm), it is preferable as a source of light for use in an image display apparatus such as a liquid crystal projector device.
It is an ultra high pressure mercury lamp of 70 Watts (W) to 280 W class that has been employed in an image display apparatus of projection type.
By the way, a pressure inside a light emitting tube must be held more highly than a predetermined pressure in order to exploit the ultra high pressure mercury lamp to the maximum performance. For this reason, even the coldest spot of the light emitting tube has to be held at a high temperature. On the other hand, the highest temperature of the light emitting tube is equal to a predetermined temperature or less in order to realize a stabilized long-term operation of the lamp. This is because crystallization of a quartz glass constituting the light emitting tube has to be prevented.
In other words, the ultra high pressure mercury lamp has to be appropriately cooled so that the temperature of the light emitting tube falls within a predetermined temperature range.
In order to cool an ultra high pressure mercury lamp in a conventional light source apparatus, two cooling methods are adopted depending on a rating of an ultra high pressure mercury lamp (applied electric power). A first cooling method is adopted in a comparatively small light source apparatus operating mainly at an electric power of 150 W or lower. In such a light source apparatus, the light emitting tube is held in a concave-shaped reflector and the front opening of the concave-shaped reflector is covered with a transparent glass plate. Therefore, the light source apparatus is substantially enclosed. The light emitting tube in the light source apparatus is cooled by blowing air to the entire of the reflector from outside and thus kept at a predetermined temperature.
A second cooling method is adopted in a comparatively large-scale light source apparatus operating mainly at an electric power 150 W or higher. In such a light source apparatus, the light emitting tube is held in a concave-shaped reflector and the front opening of the concave-shaped reflector is covered with a transparent glass plate. The light source apparatus having such a construction has a hole for cooling air to flow therethrough in a side portion of the reflector or in the center of the transparent glass plate. Therefore, the light emitting tube is directly cooled by blowing air to the inside of the reflector through the hole.
Of the two methods—one for cooling the entire of the concave-shaped reflector and the other for cooling directly the light emitting tube—the former can control the temperature of the tube in a more stabilizing way than the latter.
Japanese Patent Laid-Open Publication No. 2003-115215 discloses a light source apparatus configured in a way that the outside portion of a concave-shaped reflector is cooled.
In this light source apparatus, a light emitting tube 101 is installed in a concave-shaped reflector 102 as shown in FIG. 1. One end of the light emitting tube 101 goes through a bore established in the deepest portion of the concave-shaped reflector 102 so as to protrude outward. Also, the end portion is fixedly supported by means of a lamp base 121. The concave-shaped reflector 102 is a concave mirror having an open front side and an inner circumference face thereof serves as a reflection face. The front side of the concave-shaped reflector 102 is closed by means of a transparent glass plate 103.
On the upper outside of the concave-shaped reflector 102, the upper outside being adjacent to the upper part of the light emitting tube 101, a ventilation duct 104 is provided in order to guide cooling air. Cooling air caused by an intake fan (not shown) passes through the ventilation duct 104, to locally cool the upper outside portion of the concave-shaped reflector 102.
By the way, FIGS. 1A and 1B are a plane and a side view of the related-art light source apparatus, respectively.
In this light source apparatus, an upper outer portion of the concave-shaped reflector 102 is cooled and thus an amount of heat radiating from the upper portion of the concave-shaped reflector 102 to the portion that is hottest in the light emitting tube 101 is reduced. In addition, an inner convection caused inside of the concave-shaped reflector 102 increases a heat dissipation from the light emitting tube 101. Therefore the upper portion of the light emitting tube 101 is kept at an appropriate temperature.
In the related-art light source apparatus, an increase in the temperature of the upper portion of the light emitting tube 101 held in the concave-shaped reflector 102 is restricted by locally cooling the upper portion of the concave-shaped reflector 102 as stated above, thereby preventing the quartz glass constituting the light emitting tube 101 from being crystallized or losing its transparency.
However, not only the temperature of the upper portion of a light emitting tube but also the temperature in the sealing portion that seals discharge electrodes in the light emitting tube is important as a factor that determines the service life of an ultra high pressure mercury lamp.
An electrode portion of the light emitting tube is composed of the discharge electrodes essentially consisting of tungsten, a molybdic foil for firmly attaching the discharge electrodes to a bulb made of quartz glass, and an outer terminal. The temperature of a portion in which the molybdic foil meets the quartz glass is a significant factor for determining the life of the ultra high pressure mercury lamp.
On the other hand, when the temperature of the sealing portion that seals the discharge electrodes in the light emitting tube is reduced excessively due to the cooling, the pressure inside of the light emitting tube is accordingly reduced. As a result, luminous flux with a desired intensity cannot be obtained and intensity distribution of the flux is changed, thereby deteriorating a color balance of the image created by the image display apparatus.
On the contrary, when the temperature of the sealing portion is raised excessively due to poor cooling, oxidation of the molybdic foil used in the portion is promoted and thus adhesion between the foil and the quartz glass constituting the light emitting tube is degraded, thereby presumably causing fracture or breakage in the tube.